1. Field of the Invention
The invention relates to surface treatment technology for biomedical devices, and in particular, methods of altering the hydrophobic or hydrophilic nature of the polymeric surface of an ophthalmic lens such as a contact lens. In one preferred embodiment, this invention relates to methods of treating biomedical devices, such as contact lenses, to increase the hydrophilicity of the surface.
2. Description of the Related Art
Many devices and materials used in various biomedical applications require certain properties in the bulk of the device or material with distinct and separate properties required for the surface. For example, contact lenses preferably have high oxygen permeability through the lens to maintain good corneal health, but the materials which typically exhibit exceptionally high oxygen permeability (e.g. polysiloxanes) are hydrophobic and will adhere to the eye. Thus, a contact lens may have a core or bulk material which is highly oxygen permeable and hydrophobic, and a surface which has been treated or coated to increase the hydrophilicity, thereby allowing the lens to freely move on the eye.
In order to modify the hydrophilicity of the a relatively hydrophobic contact lens material, a contact lens may be treated with a plasma treatment. A high quality plasma treatment technique is disclosed in PCT Publication No. WO 96/31792 by inventors Nicolson, et al. However, some plasma treatment processes require significant investment in equipment. Moreover, plasma treatment requires that the lens be dry before exposure to the plasma. Thus, lenses which are wet from prior hydration or extraction processes must be dried, imposing costs of drying equipment and adding time to the overall lens production process. Accordingly, there remains a need for an inexpensive method of consistently and permanently altering the surface properties of polymeric biomaterials, especially ophthalmic lenses such as contact lenses. A particularly preferred method would be one which could be used directly on wet lenses, i.e., without requiring a preliminary drying step.
In contrast to the plasma surface treatment methods used in the ophthalmic lens art, a number of techniques have been used to treat the surface of electronic devices, thin film sensors and the like. These techniques include Langmuir-Blodgett deposition, controlled spin casting, chemisorptions and vapor deposition. Useful examples of Langmuir-Blodgett layer systems are disclosed in U.S. Pat. Nos. 4,941,997; 4,973,429 and 5,068,318 issued to Decher, et al., and assigned to Ciba-Geigy Corporation. A more recent technique used on electronic devices is a layer-by-layer polymer adsorption process which is described in “Investigations of New Self-Assembled Multilayer Thin Films Based on Alternately Adsorbed Layers of Polyelectrolytes and Functional Dye Molecules” by Dongsik Yoo, et al. (1996).
The Yoo, et al. process involved alternatively dipping hydrophilic glass substrates in a polyelectrolyte solution (e.g., polycations such as polyallylamine or polyethyleneimine) and an oppositely charged dye solution to form electrically conducting thin films and light-emitting diodes (LEDs). After each dipping, the substrates were rinsed with acidic aqueous solutions. Both the dipping and rinsing solutions had a pH of 2.5 to 7. Prior to dipping, the surfaces of the glass substrates were treated in order to create a surface having an affinity for the polyelectrolyte.
Similarly, two 1995 publications entitled “Molecular-Level Processing of Conjugated Polymers” by Fou and Rubner and by Ferreira and Rubner, describe similar methods of treating glass substrates which have hydrophilic, hydrophobic, negatively or positively charged surfaces. The glass surfaces are treated in hot acid baths followed by hot peroxide/ammonia baths for extended periods to produce a hydrophilic surface. Hydrophobic surfaces are produced by gas-phase treatment in 1,1,1,3,3,3-hexamethyldisilazane for 36 hours. Charged surfaces were prepared by covalently anchoring charges onto the surface of the hydrophilic slides. For example, positively charged surfaces were made by further treating the hydrophilic surfaces in methanol, methanol/toluene and pure toluene rinses followed by immersion in (N-2-aminoethyl-3-aminopropyl) trimethyloxysilane solution for 12-15 hours. This procedure produced glass slides with amine functionalities, which are positively charged at low pH. All of the substrate surface preparations require chemical processing and are time consuming.
U.S. Pat. Nos. 5,518,767 and 5,536,573 issued to Rubner, et al. and assigned to Massachusetts Institute of Technology, describe methods of producing bilayers of p-type doped electrically conductive polycationic polymers and polyanions or water-soluble, non-ionic polymers on glass substrates. Extensive chemical pretreatments of the glass substrates, which are the same or similar to those taught in the aforementioned articles, are described in the '767 and '573 patents.
The layer-by-layer polyelectrolyte deposition methods described in patent and literature references relate generally to production of electronic devices and treatment of rigid glass substrates. Notably, the teachings indicate that complex and time-consuming pretreatment of the substrate is required to produce a surface having a highly charged, hydrophilic or hydrophobic nature in order to bind the polycationic or polyanionic material to the glass substrate.